In known processes for filtration and stabilization of media containing tanning agents and/or protein, e.g. beer, wine, or fruit juice, the medium is first filtered in a diatomite filter (e.g. a layer filter or a multiple tube filter), thereafter stabilized in a stabilizer installation, and finally filtered once more. Such processes are correspondingly costly and require sizable investments in the installations.
Cross-flow filtration processes for microfiltration or ultrafiltration are known and common in the most diverse areas of the technology. In the cross-flow technology, the filtration membranes are not only penetrated from the unfiltered or feed side to the filtrate resp. permeate side, but on the unfiltered side, a strong cross-flow is additionally generated along the membrane surface. The purpose of the cross-flow is to prevent the formation of a certain layer of retained sludge on the side of the unfiltered medium. The transfer pumping of the unfiltered medium serves to remove the build-up of slurry substances from the membrane before the latter is blocked by them. At least, there should be a limitation of the thickness of the layer of separated substances on the membrane surface, so that filtration with a relatively constant flow of permeate should be possible over longer periods of time. However, since the separation of slurry and filtrate takes place immediately at the membrane surface and under the influence of the transmembrane pressure resp. the trans-membrane flow, the accumulation of separated slurry substances cannot be completely prevented, not even in cross-flow filtration. Mostly, a delicate cover layer of separated slurry substances will form relatively rapidly, i.e. immediately as the filtration begins, and that layer may in itself have certain filtration characteristics. Such cover layers are formed at ultrafiltration membranes with a pore dimension of approximately 0.001 to 1 .mu., as well as at microfiltration membranes with a pore dimension of approx. 0.1 to 1 .mu.. Practical experience shows that with a fine-pore surface, i.e. in the case of ultrafiltration as well, the membranes used in the cross-flow process are less susceptible to clogging than is the case with coarse-pore membrane surfaces.
In many known cross-flow filtration processes, there is consequently a periodical backwashing, i.e., during the filtration, the membranes are briefly subjected to pressure from the filtrate side, against the direction of the filtration. This causes a partial loosening or rejection of the layer on the side of the unfiltered medium, or a clearing of the membrane pores. This makes it possible to reduce the flow resistance in the filtration direction to a certain degree.